1. Field of the Invention
This invention relates to improved ink receptive film formulations, and to visual transparencies coated therewith which yield images exhibiting decreased fading of triarylmethane dyes. The transparencies comprise a substantially transparent film backing, and an image-receptive coating thereon which includes specific protic, hydrogen bonding organic-solvent-soluble additives.
b 2. Description of the Related Art
Ink jet printing is an established technique for printing variable information such as labels, multi-color graphics, and the like. Presentation of such information has created a demand for transparent polymeric films which are image-receptive for viewing in a transmission mode. One technique for preparing such articles, commonly known as "visuals" or "transparencies", involves depositing ink on the surface of the sheet to provide the desired images. Recently, computer driven graphic plotting devices have become available which can quickly and precisely generate complex, graphic information. Movement of the ink jet may be computer controlled, and information is therefore printed at electronic speeds.
The graphic plotting devices can generate particularly attractive and effective materials for visual presentation. These plotters conventionally utilize pens containing solvent-based inks which can remain exposed to the air for long periods of time without drying out. However, the nature of the inks required to maintain reasonably long pen life, e.g., low volatility, poses problems in the preparation and handling of the transparencies.
When printing on paper by means of ink jet printers, the images are composed of small dots being spaced to allow diffusion of the ink to cover the area between dots. When printing on film, there is little or no ink spreading. Also, many substrates do not readily accept those inks and the ink will bead up on the surface of the film.
Problems with transparent films also include failure to dry quickly. Some substrates which accept the inks to a greater degree require an extended period of time, e.g., three minutes or more, before the surface is dry enough to handle. This greatly increases the likelihood that the image will be smeared during removal of the transparency from the plotter, or during subsequent handling and stacking of the imaged sheets. Other inking problems are irregular image density, including dots at the end of a pen stroke and severe striations resulting from the multiple, adjacent pen strokes required to "paint" a large block of color, such as when generating a colored bar or pie chart. To help solve these problems, polymeric sheets are usually coated with a transparent liquid-absorbent layer capable of absorbing large quantities of liquid. In addition to pre-imaging requirements, polymeric blends used in ink-receptive layers must also exhibit satisfactory post-imaging quality, such as quick drying and the like.
It is desirable that transparent liquid-absorbing polymeric compositions retain some degree of physical durability, dryness to the touch, and non-tackiness after absorbing significant amounts of water, as would happen during imaging with aqueous inks. Because polymers typically are significantly softened or even dissolved by the absorption of liquids, the required goals of absorption and durability are inconsistent. Attempts at resolving these conflicting goals have resulted in the use of polymer blends.
Compositions useful as transparent liquid-absorbent materials have been formed by blending a liquid-insoluble polymer material with a liquid-soluble polymeric material. The liquid-insoluble material is presumed to form a matrix, within which the liquid-soluble materials reside. Examples of such blends are disclosed in U.S. Pat. Nos. 4,300,820, and 4,369,229, wherein the matrix-forming polymer is a terpolymer comprised of hydrophobic monomeric units, hydrophilic monomeric units, and acid-containing monomeric units, and the liquid-soluble portions of the compositions are polyvinyl lactams. Although these patents do not disclose ink-receptive coatings, the blends disclosed have been found useful as water-absorbent layers capable of retaining a degree of durability.
Other examples of such blends are disclosed in European Patent Application No. EP 0 233 703, wherein water-insoluble acrylic polymers having acid functionality are blended with water-soluble polyvinyl lactams, e.g., polyvinyl pyrrolidone for use as ink-receptive layers on films to be imaged by ink jet printers or pen plotters. However, these formulations do not simultaneously provide adequate drying, low tack, and other required properties when used in many of the commercially available ink jet printers.
U.S. Pat. No. 4,935,307 discloses a hydrophilic polymeric blend which provides improved durability and reduced curl. The blend comprises at least one water-absorbing, hydrophilic polymeric material, at least one hydrophobic polymeric material having acid functionality, and at least one polyethylene glycol.
An additional problem in using various blends of liquid-absorbent polymers is the incompatibility of the matrix-forming insoluble polymer with the liquid being absorbed. For example, if the liquid being absorbed is water, and if the water-insoluble polymers are hydrophobic, some inhibition of water absorption ability can be expected. One method of overcoming this difficulty is to utilize hydrophilic matrix polymers that are not water-soluble at the use temperature, but are water-soluble at other temperatures.
U.S. Pat. No. 4,503,111 discloses ink-receptive coatings comprising either polyvinyl alcohol or gelatin blended with polyvinyl pyrrolidone. Both are water-insoluble at room temperature, are able to act as matrix-forming polymers for these coatings, and the coatings are quite receptive to aqueous inks.
However, the coatings do exhibit a tendency to become tacky, either because of imaging, or because of high humidity. U.S. Pat. Nos. 4,225,652 and 4,379,804 (Eisele), disclose visual transparencies comprising a liquid-absorbent underlayer, and a liquid-permeable protective overlayer. The liquid sorptivity of the underlayer is greater than the liquid sorptivity of the surface layer.
Another problem associated with the use of transparency films with liquid-absorbing coatings is that the images made using certain inks from pen plotters and ink-jet printers are not storage stable when imaged onto such visual transparencies in the ink-receptive layers. Dark fading, and other distortions of the image color occur after imaging, especially after a period of time has elapsed. Triarylmethane dyes are used in inks for graphic printers and plotter devices. When they are imaged onto either single or multiple transparencies having substantially aprotic characteristics, they tend to react with nucleophilic agents already present. This causes the image to fade or bleach out over time, rendering the image unacceptable for viewing. This fade may cause portions of the image to appear bleached and others to have a distorted color.
It has now been discovered that this fading can be substantially reduced, or even eliminated by the addition of certain protic, hydrogen-bonding, organic-solvent-soluble additives to the formulation of the transparent liquid-absorbent layer of a transparency while maintaining the other required characteristics of a visual transparency such as quick drying, dimensional stability and the like.